In recent years, a porous honeycomb ceramics filter has been used as an apparatus for removing particulates in exhaust gas, including a structure in which a plurality of through holes opened in end surfaces on exhaust gas inflow and outflow sides are alternately closed in the opposite end surfaces. The exhaust gas which has flown in from the end surface on the exhaust gas inflow side is forcibly passed through partition walls (including a plurality of fine pores) among the respective through holes to trap/remove the particulates in the exhaust gas.
For the ceramics filter, a pore diameter of the fine pore formed in the partition wall between the respective through holes is determined by a relation with a particulate diameter of the particulate in the exhaust gas, and capabilities such as trapping efficiency and pressure loss differ with a degree of the pore diameter. In view of this, development has heretofore been advanced so as to obtain the ceramics filter having a high trapping efficiency of particulates (such as soot).
However, for the filter having the raised trapping efficiency, since the pore diameter of the filter is small, there is a problem that an ash content generated by oil combustion of a diesel engine is accumulated in the pores of the filter and the pressure loss is raised to be not less than a predetermined loss by the use for a long period. For the filter having a small pore diameter, the pressure loss is high in sampling the particulates (such as the soot), and there has been a demand for reduction of the loss.
Additionally, in recent years, by improvement of the diesel engine, the particulates such as the soot generated from the engine have been reduced by a considerable amount than before. Then, as a result of the present inventor's intensive studies, in order to clear a regulated value of a future exhaust gas standard, it is not necessary to achieve an extremely high trapping efficiency. Conversely, it has been found that in order to prevent a rise of the pressure loss caused by the accumulation of the ash content into the pores, it is important to increase the pore diameter to such an extent that the exhaust gas can pass through the partition wall, and the present invention has been attained.
Therefore, an object of the present invention is to provide a honeycomb ceramics filter in which a predetermined trapping efficiency is maintained, an ash content generated by oil combustion of a diesel engine is extracted from pores, and pressure loss is accordingly prevented from increasing to be not less than a predetermined loss even with long-period use.